1. Field of the Invention
The present invention relates to a color image forming apparatus and, more particularly, to a tandem color image forming apparatus equipped with plural color development means and transfer means for sequentially transferring images of plural colors formed by the plural color development means.
2. Description of the Related Art
In recent years, tandem color image forming apparatus have become widespread to create images at higher speed by electrophotographic color image forming apparatus. A tandem color image forming apparatus has as many developing units and photoconductor drums as color materials, an images of different colors are successively transferred onto a transfer belt and onto a recording medium. It is already known that plural factors cause misregistration in such a tandem color image forming apparatus. Various methods have been proposed to cope with each of such factors. One of the factors is that lenses used in a deflection and scanning device are not uniform. Another factor is that the lenses deviate from their proper installation positions. A further factor is that the position at which the deflection and scanning device is installed to the body of the color image forming apparatus deviates from its proper position. In this case, each scanning line is tilted or curved. The degree of the tilt or curvature differs according to different color. Consequently, misregistration takes place.
A method of coping with the misregistration is described in Japanese Patent Laid-Open No. 2002-116394. In particular, during the step of assembling a deflection and scanning device, the magnitude of the curvature of each scanning line is measured using an optical sensor. The lens is mechanically rotated to adjust the curvature of the scanning line. Then, the lens is bonded with adhesive.
Japanese Patent Laid-Open No. 2003-241131 describes a method of installing a deflection and scanning device to the body of a color image forming apparatus by measuring the degree of the tilt of each scanning line using an optical sensor during the step of installing the deflection and scanning device to the body, mechanically tilting the deflection and scanning device to adjust the tilt of the scanning line.
Japanese Patent Laid-Open No. 2004-170755 describes a method of creating a corrected image by measuring the tilt and the magnitude of the curvature of each scanning line using an optical sensor and correcting bitmap image data such that the tilt and curvature are canceled out.
The method described in Japanese Patent Laid-Open No. 2004-170755 makes corrections by image-processing image data and, therefore, mechanical adjusting members and an adjusting step normally performed during assembly are dispensed with. In these respects, the method can cope with misregistration more economically than the methods described in the above-cited Japanese Patent Laid-Open Nos. 2002-116394 and 2003-241131.
The correction to misregistration using the image processing is carried out when the scanning beam deviates in the vertical scanning direction as shown in FIG. 12. In the method described in the above-cited Japanese Patent Laid-Open No. 2004-170755, the tilt and curvature of each scanning line in the outputted matter are canceling out by creating a shifted image from the original image at each position (herein may be referred to as the scan line changing point) like an image obtained by scan line changing processing illustrated in FIG. 13.
However, the aforementioned conventional example has the following problems. First, a bitmap image printed with a printer is corrected by image processing as a premise. Therefore, it is necessary to form a bitmap image complying with the tilt and curvature of the optical system of the print engine.
At this time, the image is shifted in increments of pixels in a corresponding manner to scan line changing points. Consequently, there is the problem that jaggies or boundaries are produced at locations corresponding to the scan line changing points in the printout.
Jaggies and boundaries are also produced even at 1/2 speed and 1/3 speed obtained by varying the process speed. Hence, it has been impossible to produce good output images.
A method consisting of creating an image at enhanced resolution is available to solve the foregoing problem. Where the resolution is enhanced, a large capacity of memory is necessary to process the enhanced resolution. This produces various problems including increased costs and performance deterioration.